Self-configuring interface architecture on flash memories

ABSTRACT

A low-power interface for nonvolatile writeable memory is described. The interface includes an input buffer and an output buffer. The input buffer receives input signals having one of a number of pairs of logic levels. The input buffer is coupled to the nonvolatile writeable memory and coupled to the same power supply as the nonvolatile writeable memory. The input buffer translates the input signals received to the signal level used by the nonvolatile writeable memory. The output buffer is coupled to the nonvolatile writeable memory and is coupled to a different power supply from the input buffer and the nonvolatile writeable memory. The output buffer translates the signals received from the nonvolatile writeable memory to the same signal levels as the input signal. The input buffer and output buffer utilize input/output signals having logic levels compatible with complementary metal-oxide semiconductor (CMOS) technology.

FIELD OF THE INVENTION

This invention relates to systems including nonvolatile writeablememory. More particularly, this invention relates to interfacing anonvolatile writeable memory device to an electronic system application.

BACKGROUND OF THE INVENTION

Many computing systems such as personal computers, automotive andairplane control, cellular phones, digital cameras, and handheldcommunication devices use nonvolatile writeable memories to store eitherdata, or code, or both. Such nonvolatile writeable memories includeElectrically Erasable Programmable Read-Only Memories ("EEPROMs") andflash Erasable and Electrically Programmable Read-Only Memories ("flashEPROMs" or "flash memories"). Nonvolatility is advantageous for allowingthe computing system to retain its data and code when power is removedfrom the computing system. Thus, if the system is turned off or if thereis a power failure, there is no loss of code or data.

The nonvolatile writeable memories often include a plurality ofinterconnected very large scale integration (VLSI) circuits. These VLSIcircuits dissipate power in proportion to the nominal voltage swing ofthe binary signals applied to the circuits. The industry standard VLSIcomplementary metal-oxide-semiconductor (CMOS) circuits currentlyutilize two levels of input/output (I/O) signals, 1.8 volts and 3.0volts. Generally, in those circuits utilizing the 1.8 volt signal level,a logic low state (logic "0") is represented by a signal level of 0volts, and a logic high state (logic "1") is represented by a signallevel of 1.8 volts. Generally, in those circuits utilizing the 3.0 voltsignal level, a logic low state (logic "0") is represented by a signallevel of 0 volts, and a logic high state (logic "1") is represented by asignal level of 3.0 volts. Therefore, the VLSI CMOS circuits areattractive for use in digital circuits because of lower powerconsumption. As the rail-to-rail voltage swing of standard CMOS circuitsutilizing the 3.0 volt signal level tends to cause such circuits todissipate excessive amounts of power and energy over CMOS circuitsutilizing the 1.8 volt signal level, the 1.8 volt CMOS circuit would bepreferred in an application requiring reduced power consumption.

With the size of many electronic products becoming increasingly smaller,many electronic product designers are currently seeking to minimizepower consumption. Generally, reducing the overall magnitude ofrail-to-rail voltage swings of CMOS circuits allows a reduction in powerconsumption. Thus, an electronic architecture that would allow and workwith lower input voltage swings without drawing leakage current isdesirable. However, certain applications of CMOS circuits are actuallymore efficient in terms of power consumption when operated at highersignal levels. For example, CMOS circuits configured as nonvolatilewriteable memory core circuits have better power efficiency whenoperated at the 3.0 volt I/O signal level and supply voltages comparedto those operated at the 1.8 volt I/O signal level and supply voltages.This increased efficiency at the higher I/O signal voltage level is aresult of the charge pumps required by the nonvolatile writeable memory.Consequently, an electronic system architectural concept is desiredwhereby the nonvolatile writeable memory circuits would be allowed tooperate with industry standard 1.8 volt and 3.0 volt CMOS I/O signallevels and utilize the optimum core supply voltage for the nonvolatilewriteable memory core circuits.

Designers of prior art electronic systems incorporating nonvolatilewriteable memory have attempted to reduce the overall system powerconsumption by running the entire system at the 1.8 volt I/O signallevel and supply voltage. This increases the power consumptionefficiency of the system exclusive of the nonvolatile writeable memory.However, the nonvolatile writeable memory core memory circuits runningat the 1.8 volt I/O signal level have a reduced power consumptionefficiency. Thus, to effectively maximize efficiency of the overallelectronic system, I/O interface buffers are required which allow thenonvolatile writeable memory core memory circuits to be operated at a3.0 volt I/O signal level, while the surrounding system CMOS circuitryis operated at a 1.8 volt I/O signal level. The 3.0 volt I/O nominalsignal level can be approximately in the range 2.7 volts to 3.6 volts.

Designers of prior art I/O interface circuitry have attempted to use 1.8volt I/O signal level buffers while running the nonvolatile writeablememory core memory circuits at a 3.0 volt I/O signal level. Regardingthe input buffer portion of the I/O interface, the prior art CMOS inputbuffers have the input high signal level equal to or within sometolerable specifications to a supply voltage. For the 1.8 volt I/Osignal level input buffers, the input high value is substantially lowerthan the input buffer supply voltage which is typically 3.0 volts. Thisdifference between the input buffer supply voltage, which is also thecore supply voltage, and input high voltage signal level, is the sourceof current leakage and unstable operation of the input buffer.

Furthermore, this current leakage problem limits the flexibility of useof the I/O interface circuitry. This is because anytime there is adifference between the supply voltage and the input high voltage signallevel there will be current leakage. Therefore, an input bufferconfigured to operate with a 1.8 volt I/O signal level cannot be used ina system utilizing 3.0 volt I/O signal levels, and vice versa. Thisrequires separate input buffer configurations to be made available foruse in each of the 1.8 and 3.0 volt I/O signal level systems. Moreover,the user does not have the option to run at the higher 3.0 volt CMOSinput signal level once a circuit is configured to operate at the 1.8volt input signal level.

Designers of prior art I/O interface circuitry have attempted to solvethis current leakage problem by using one power supply for the 1.8 voltinput buffer and a separate power supply for the non-volatile writeablememory core memory circuits operating at the 3.0 volt signal level. Thisis problematic in that the limits of size and weight imposed by manyelectronic applications using nonvolatile writeable memory circuits donot allow for the use of more than one power supply.

Regarding the output buffer portion of the I/O interface, the prior artCMOS output buffers have p-channel CMOS drivers, or voltage levelpull-ups, driving the output high level equal to or within sometolerable specifications to the supply voltage. For the 1.8 volt I/Osignal level output buffers, the lower voltage power supply limits theinternal drive capability to meet higher output speed and loadrequirements in driving a voltage output high level.

Another limitation found in prior art I/O circuitry which can have asignificant adverse impact in particular applications is the electricalnoise generated by the circuit configuration. In a prior artconfiguration using a single power supply, the interface circuitry inputbuffer, nonvolatile writeable memory core memory circuits, and theinterface circuitry output buffer of the system are all connected to thesame power supply output. In a prior art configuration using separatepower supplies for the I/Os and the nonvolatile writeable memory corecircuits, the input buffer and the output buffer are connected to thesame power supply output. Consequently, in both configurations, theisolation between the input and the output is reduced by having theinput and the output connected to the same power supply output. Thisconfiguration significantly reduces the noise immunity of the system.This problem is compounded when a system is operated at the 1.8 volt I/Osignal level because, at this signal level, the noise margin isdecreased. This noise can have significant adverse impacts onperformance, particularly in cellular phone applications.

SUMMARY OF THE INVENTION

A low-power interface for nonvolatile writeable memory is described. Thelow-power interface receives input signals having one of a number ofpairs of logic levels. The low-power interface translates the inputsignals to a pair of logic levels that may differ from the pair of logiclevels of the input signal. The low-power interface provides outputsignals at the same pair of logic levels as the input signals.

A method of handling signals associated with a nonvolatile writeablememory when the signals have a number of pairs of logic levels isdescribed. The method comprises the step of translating a first pair oflogic levels of a nonvolatile writeable memory input signal to a secondpair of logic levels when the first pair of logic levels of the inputsignal are different from the second pair of logic levels. The methodfurther comprises the step of translating a second pair of logic levelsof a nonvolatile writeable memory output signal to a first pair of logiclevels when the second pair of logic levels of the output signal aredifferent from the first pair of logic levels.

Other features and advantages of the present invention will be apparentfrom the accompanying drawings and from the detailed description andappended claims that follow below.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and notlimitation in the figures of the accompanying drawings, in which likereferences indicate similar elements and in which:

FIG. 1 shows a block diagram of a system level application includingnonvolatile writeable memory.

FIG. 2 shows a block diagram of a nonvolatile writeable memory.

FIG. 3 shows the power supply combination supplied to an embodiment of anonvolatile writeable memory.

FIG. 4 shows a schematic of an embodiment of an interface circuit inputbuffer of a nonvolatile writeable memory.

FIG. 5 shows a schematic of an embodiment of an interface circuit outputbuffer of a nonvolatile writeable memory.

FIG. 6 shows a schematic of an alternate embodiment of an interfacecircuit output buffer of a nonvolatile writeable memory.

DETAILED DESCRIPTION

Nonvolatile writeable memory interface circuits that areself-configuring to multiple CMOS input/output signal levelspecifications will provide low power consumption and prevent currentleakage at both signal levels while using a common core power supply. Alow-power interface is provided for nonvolatile writeable memory that isself-configuring to multiple CMOS input/output (I/O) signal levels.

FIG. 1 shows a block diagram of a system level application including anonvolatile writeable memory. This application includes amicrocontroller or digital signal processor 102 and system components104-108. System components 104-108 can be any other electroniccomponents of the system which, for example, might include but is notlimited to additional memory components like static random access memory(SRAM), EPROM, and EEPROM. The microcontroller 102 communicates with thenonvolatile writeable memory 110 via address lines 118 and input/output(I/O) data lines 120. A first output of a single power supply 112provides a 1.8 volt supply (VCCQ) 114 to an interface circuit of thenonvolatile writeable memory 110 as well as to the systemmicrocontroller 102, and system components 104-108. A second output ofthe power supply 112 provides a 2.7 volt supply (VCC) 116 to the corememory circuits of the nonvolatile writeable memory 110.

FIG. 2 shows a block diagram of a nonvolatile writeable memory 110. Thisembodiment depicts the nonvolatile writeable memory 110 as beingcomprised of a core memory circuit 222 and interface circuitry. The corememory circuit 222 includes, but is not limited to, a command register,a write state machine, a resolution circuit, read/write circuits, and amemory cell array (not shown). The interface circuitry is comprised of a1.8/3.0 volt input buffer 220 and a 1.8/3.0 volt output buffer 224.

An electronic system is coupled to the nonvolatile writeable memory 110with address and control lines 118 which are coupled to the input buffer220. The input buffer 220 is coupled to the core memory circuit 222. Thecore memory circuit 222 is coupled to the output buffer 224. The outputbuffer 224 is coupled to an electronic system using I/O data lines 120.

With regard to power supply connections, the input buffer 220 and thecore memory circuit 222 are each coupled to a VCC power supply output116. The output buffer 224 is coupled to a VCCQ power supply output 114.The VCC power supply output 116 and the VCCQ power supply output 114 incombination power the nonvolatile writeable memory 110 and theelectronic system in which the nonvolatile writeable memory 110 isresident.

FIG. 3 shows the power supply combination 300 supplied to an embodimentof a nonvolatile writeable memory circuit by the VCC power supply output116 and the VCCQ power supply output 114. The interface circuitry of anembodiment of the nonvolatile writeable memory circuit, including theinput buffer 220 and the output buffer 224, is self-configuring tooperate with a number of sets of signal levels. These signal levels aregenerally compatible with complementary metal-oxide semiconductor (CMOS)technology. For example, the nonvolatile writeable memory circuit canutilize a signal having either a 1.8 volt or a 3.0 volt signal level.When utilizing the 1.8 volt signal level, a logic low state (logic "0")is represented by a signal level of 0 volts, and a logic high state(logic "1") is represented by a signal level of 1.8 volts. Whenutilizing the 3.0 volt signal level, a logic low state (logic "0") isrepresented by a signal level of 0 volts, and a logic high state (logic"1") is represented by a signal level of 3.0 volts.

With reference to FIGS. 2 and 3, when the interface circuitry includingthe input buffer 220 and the output buffer 224 is configured to utilizea 1.8 volt CMOS signal 302, the VCC power supply output 116 to the inputbuffer 220 and the core memory circuit 222 is approximately in the rangeof 2.7 to 2.85 volts 306, and the VCCQ power supply output 114 to theoutput buffer 224 is approximately in the range of 1.8 to 2.2 volts 304.When the interface circuitry including the input buffer 220 and outputbuffer 224 is configured to utilize a 3.0 volt CMOS signal 312, the VCCpower supply output 116 to the input buffer 220 and the core memorycircuit 222 is substantially equal to 3.0 volts 316, and the VCCQ powersupply output 114 to the output buffer 224 is substantially equal to 3.0volts 314.

FIGS. 4, 5, and 6 show schematics of embodiments of an interface circuitinput buffer and output buffers of a nonvolatile writeable memory. Inthese figures, all transistors are of the CMOS type, with a bubble atthe gate indicating a p-channel device, a bubble in the gate indicatingwhat will be referred to herein as an S device, and the absence of abubble at the gate indicating an n-channel device. It is readilyapparent that the circuitry described herein is not limited to CMOSdevices. Furthermore, all values recited herein are approximated; it iscontemplated that values in the range about the value recited areapplicable.

FIG. 4 shows a schematic of an embodiment of an interface circuit inputbuffer 400 of a nonvolatile writeable memory. The input buffer 400receives inputs 470 in the form of addresses from an electronic systemmicrocontroller or processor. The inputs 470 are received at the gatesof two transistors 412 and 414 which form an inverter 415. Transistor412 is a p-channel device which has its drain coupled to transistor 414,an n-channel device, at node 452. Transistor 412 has its source coupledto the source of an S device 410 at node 450. The gate and drain of theS device 410 is coupled to a VCC power supply output 116 using an enableswitch device.

The S device 410 is a high transconductance n-channel transistor that isspecially doped to provide a threshold voltage that is lower than thethreshold voltage of a standard n-channel CMOS device. In oneembodiment, the S device is doped to have a threshold voltage ofapproximately 0.3 volts.

The inverter 415 is coupled to an input buffer driver 418 at node 452.The input buffer driver 418 is an inverter similar in electronicstructure to input inverter 415. The output of the input buffer driver418 is coupled to the gate of a p-channel device 416. The source of thep-channel device 416 is coupled to a VCC power supply 116. The drain ofthe p-channel device 416 is coupled to the input of the input bufferdriver 418 at node 454. The output of the input buffer driver 418 is theoutput 480 of the input buffer circuit 400.

The gate drive across a CMOS device is described relative to thedifference in the voltage present at the gate and the voltage present atthe source of the device, such voltage difference referred to as Vgs.The state of a transistor as "on" or "off" is determined by therelationship between the threshold voltage of the device, referred to asVt, and Vgs. Accordingly, the device is considered to be "on" when Vgsis greater than Vt, and the device is considered to be "off" when Vgs isless than Vt.

In analyzing the operation of input buffer 400, the input buffer 400 isautomatically configured to utilize both 1.8 volt and 3.0 volt signallevels without current leakage as a result of the S device 410. In theoperation of the input buffer 400, leakage current results when devices412 and 414 are "on", resulting in a current path from the power supplyoutput 116 to ground 499. Therefore, either device 412 or 414 must be"off" in order to prevent leakage current from flowing. For a device tobe "off", the Vgs of the device must be less than the Vt of the device.The Vt of a p-channel device like transistor 412 is approximately 1volt. Therefore, the Vgs must be less than approximately 1 volt toinsure the device is "off".

An analysis of input buffer 400 operating with a 1.8 volt input signallevel follows. In the 1.8 volt I/O configuration, the VOC power supplyoutput 116 is approximately 2.7 volts. The Vt of the S device 410 isapproximately 0.3 volts. This 0.3 volt drop applied to the 2.7 voltpower supply output 116 across the threshold of the S device 410 resultsin a voltage at node 450, the drain of device 412, of approximately 2.4volts. A logic high signal received at the input 470 results in a signallevel of approximately 1.8 volts at the gate of device 412. A gatevoltage of 1.8 volts and a source voltage of 2.4 volts results in a Vgsof approximately 0.6 volts. This Vgs of 0.6 volts is less than the Vt ofapproximately 1.0 volt for the p-channel device 412 resulting in device412 being "off". As device 412 is "off" there is no direct current pathfrom the power supply output 116 to ground 499 and thus, no leakagecurrent can flow when the input buffer 400 is operating with 1.8 voltI/O signal levels.

An analysis of input buffer 400 operating with a 3.0 volt input signallevel follows. In the 3.0 volt I/O configuration, the VOC power supplyoutput 116 is approximately 3.0 volts. The Vt of the S device isapproximately 0.3 volts. This 0.3 volt drop applied to the 3.0 voltpower supply output 116 across the threshold of the S device 410 resultsin a voltage at node 450 of approximately 2.7 volts. A logic highvoltage signal received at the input 470 results in a signal level ofapproximately 3.0 volts at the gate of device 412. A gate voltage of 3.0volts and a source voltage of 2.7 volts results in a Vgs ofapproximately 0.3 volts. This Vgs of 0.3 volts is less than the Vt ofapproximately 1.0 volt for the p-channel device 412 resulting in device412 being "off". As device 412 is "off" there is no direct current pathfrom the power supply output 116 to ground 499 and thus, no leakagecurrent can flow when the input buffer 400 is operating with 3.0 voltI/O signal levels.

Therefore, the input buffer 400 can be operated at both 1.8 volt and 3.0volt I/O signal levels without any required reconfiguration by the user;the input buffer 400 automatically responds to either input signal levelconfiguration. While operating at either a 1.8 volt or a 3.0 volt inputsignal level, the input buffer 400 functions with the same power supplyoutput 116 voltage as the nonvolatile writeable memory core memorycircuits. While operating at either signal level, there is no currentleakage because of the special threshold voltage of the S device 410.The S device 410 threshold voltage drops the VCC supply output 116voltage to the input buffer 400 inverter 415 such that when the input470 is in a logic high state, all of the transistors in the input bufferstage are "off". The choice of this special threshold S device 410 iscritical because the voltage drop it induces as a result of itsthreshold voltage turns the p-channel device 412 in the input buffer 400completely "off" within the range of the input voltage specifications atboth the 1.8 volt and 3.0 volt signal levels.

The input buffer driver 418 and feedback pull-up device 416 of the inputbuffer 400 function to translate the level of the signal received at theinput 470 to the internal operating signal level of the nonvolatilewriteable memory core memory circuits, approximately 2.7 volts. Theinput buffer driver 418 has an electrical configuration similar toinverter 415, being comprised of an n-channel and a p-channel device.Consequently, the same problem regarding leakage current as previouslydiscussed with regard to inverter 415 applies to the input buffer driver418.

As shown in the previous analysis, the voltage at node 450 is 2.4 voltswhen the input buffer 400 is operated at a 1.8 volt I/O signal level.This results in a voltage of 2.4 volts at nodes 452 and 454 when a logichigh signal level is received at the input 470. This 2.4 volt signal isfed back through the input buffer driver 418 as a 0 volt signal. The 0volt signal is applied to the gate of transistor 416. As the source oftransistor 416 is coupled to the VCC power supply output 116, thevoltage present at the source of transistor 416 is approximately 2.7volts. Therefore, the Vgs of transistor 416 is 2.7 volts which isgreater than the approximately 1.0 volt Vt of a p-channel deviceresulting in transistor 416 being turned "on". With transistor 416 "on",the voltage at node 454 is pulled up to 2.7 volts. As the source of thep-channel device of the input buffer driver 418 is coupled to a 2.7 voltVCC power supply output, the presence of a 2.7 volt signal level at theinput of the input buffer driver 418 results in a Vgs of the p-channeldevice of the input buffer driver 418 of 0 volts which turns the device"off". As the device is "off" there is no direct current path from thepower supply to ground so that no leakage current can flow in the inputbuffer driver 418 when the input buffer 400 is operated at a 1.8 voltI/O signal level.

As shown in the previous analysis, the voltage at node 450 is 2.7 voltswhen the input buffer 400 is operated at a 3.0 volt I/O signal level.This results in a voltage of 2.7 volts at nodes 452 and 454 when a logichigh signal level is received at the input 470. This 2.7 volt signal isfed back through the input buffer driver 418 as a 0 volt signal. The 0volt signal is applied to the gate of transistor 416. As the source oftransistor 416 is coupled to the VCC power supply output 116, thevoltage present at the source of transistor 416 is approximately 2.7volts. Therefore, the Vgs of transistor 416 is 2.7 volts which isgreater than the approximately 1.0 volt Vt of a p-channel deviceresulting in transistor 416 being turned "on". With transistor 416 "on",the voltage at node 454 is maintained at 2.7 volts. As the source of thep-channel device of the input buffer driver 418 is coupled to a 2.7 voltVCC power supply output, the presence of a 2.7 volt signal level at theinput of the input buffer driver 418 results in a Vgs of the p-channeldevice of the input buffer driver 418 of 0 volts which turns the device"off". As the device is "off" there is no direct current path from thepower supply to ground so that no leakage current can flow in the inputbuffer driver 418 when the input buffer 400 is operated at a 3.0 voltI/O signal level.

FIG. 5 shows a schematic of an embodiment of an interface circuit outputbuffer 500 of a nonvolatile writeable memory. The output buffer 500receives inputs 506 in the form of data from a nonvolatile writeablememory core memory circuit. The inputs are received at the gates of twoinverters 502 and 504. Each of the two inverters 502 and 504 arecomprised of an n-channel and a p-channel device coupled together andcoupled to a VCC power supply output 116, as previously discussed. Theinverters 502 and 504 each function as previously discussed with regardto inverter 415 of the input buffer 400. Inverter 504 is coupled to thegate of an n-channel transistor 509 at node 530. The source oftransistor 509 is coupled to the output 508 of the output buffer 500.Inverter 504 drives the voltage output low level signal with transistor509.

Inverter 502 is coupled to the gate of a p-channel transistor 516 atnode 532. Inverter 502 is also coupled to the input of inverter 503 atnode 532. The output of inverter 503 is coupled to the gate of an Sdevice 510. The characteristics of the S device 510 are the same aspreviously discussed for S devices. The sources of both the p-channeltransistor 516 and the S device 510 are coupled to a VCCQ power supplyoutput 114. The VCCQ power supply output 114 is as previously discussed.The drains of both the p-channel transistor 516 and the S device 510 arecoupled to the output 508 of the output buffer 500. Inverter 502 drivesthe voltage output high level signal with the parallel combination ofthe p-channel transistor 516 and the S device 510.

An analysis of output buffer 500 operating with 1.8 volt I/O signallevels follows. In the 1.8 volt I/O configuration, the VCC power supplyoutput 116 is approximately 2.7 volts. A logic high state signal at theinput 506 of the output buffer 500 results in a logic low state signalat node 532, at the gate of the p-channel transistor 516, and at theinput of inverter 503. The logic low state signal at the gate of thep-channel transistor 516 turns the p-channel transistor 516 "on" andallows the VCCQ power supply output 114 to drive the output 508 throughthe p-channel transistor 516. Simultaneously, the logic low state signalat the input of inverter 503 is translated into a logic high statesignal by the inverter 503. The logic high state signal is coupled tothe gate of the S device 510 thereby turning the S device 510 "on" andallowing the VCCQ power supply output 114 to simultaneously drive theoutput 508 through the S device 510.

In analyzing the operation of the output buffer 500 in driving theoutput 508 with a 1.8 volt signal, the p-channel transistor 516 and theS device 510 are coupled to the VCCQ power supply output 114 whichprovides a voltage level of approximately 1.8 volts. As a result ofcapacitive effects, the p-channel transistor 516 alone is limited in itsinternal drive capability to meet the higher output speed and loadrequirements associated with driving the 1.8 volt voltage output highlevel. This is because the high threshold voltage (approximately 1 volt)of the p-channel transistor 516 limits the driving potential of a 1.8volt power source to approximately 0.8 volts. Thus, the hightransconductance and low threshold voltage (approximately 0.3 volts) ofthe specially doped S device 510 are required to drive the outputvoltage to a high level of approximately 1.5 volts as fast as possible.After reaching a voltage level of approximately 1.5 volts, the S device510 cuts off. The S device 510 cutoff allows the parallel p-channeltransistor 516 to drive and sustain the remaining voltage output highlevel of 1.8 volts.

The separate VCC 116 and VCCQ 114 power supply output connections to theoutput buffer 500 enable the output buffer 500 to operate with both 1.8volt and 3.0 volt CMOS I/O signal levels. Furthermore, the separateconnections 116 and 114 allow the nonvolatile writeable memory corememory circuit supply voltage to be independent from the output buffer500 supply voltage. Thus, the separate power supply connections allowfor the highest possible noise immunity and isolation between the inputand output buffers which, when using lower I/O signal voltage levels, issignificant because of decreased noise margins.

FIG. 6 shows a schematic of an alternate embodiment of an interfacecircuit output buffer 600 of a nonvolatile writeable memory. Theinverters 502 and 504 and the parallel combination of the p-channeltransistor 516 and the S device 510 have the same functions aspreviously described with regard to the output buffer 500 of FIG. 5.However, because of process skews in the circuit manufacturing process,it is desirable to be able to trim a circuit for use in numerousparticular applications. The additional components 602-612 of the outputbuffer 600 allow the output buffer 600 to be optimized for speed andnoise in a particular application.

The additional components 602-612 of output buffer 600 comprise twoadditional S devices 602 and 604 in parallel with S device 510. Two Sdevices are shown for example only, and the number of S devices used inparallel with S device 510 is not limited to two. The additional Sdevices 602 and 604 increase the rate at which the output signal reachesthe level of the VCCQ power supply output 114. Consequently, the more Sdevices in parallel, the faster the output will reach the level of theVCCQ power supply output 114.

Furthermore, the additional components of the output buffer 600 comprisethe NOR logic gates 606 and 608, the content addressable memory 610, andthe switch 612. These components 608-612 allow for selective activationof the additional S devices 602 and 604, thereby allowing for selectivetrimming for particular circuit applications. The content addressablememory 610 is comprised of a number of nonvolatile writeable memory corememory cells. The contents of these core memory cells allow forselective coupling of the output of inverter 502 to the gates of theadditional S devices 602 and 604 by use of the NOR logic gates 606 and608. Therefore, if a particular circuit application results in avariance in the capacitive or inductive effects of the circuit, thenumber of S devices used in the output buffer of that particularapplication can be programmably altered to overcome the adverse affects.Accordingly, the speed and noise parameters of the output buffer can bemaintained as a relative constant between applications. Thus, alow-power interface for nonvolatile writeable memory that isself-configuring to multiple CMOS I/O signal levels has been provided.

Although the detailed description describes embodiments using a flashEPROM, the invention may be used with any nonvolatile writeable memory,including, but not limited to, EPROMs, EEPROMs, and flash memories,including technologies such as NOR, NAND, AND, Divided bit-line NOR(DINOR), and Ferro-electric Random Access Memory (FRAM). While thepresent invention has been described with reference to specificexemplary embodiments, it will be evident that various modifications andchanges may be made to these embodiments without departing from thebroader spirit and scope of the invention as set forth in the claims.Accordingly, the specification and drawings are to be regarded in anillustrative rather than a restrictive sense.

What is claimed is:
 1. A nonvolatile memory having a low-power interfacethat receives input signals having one of a plurality of pairs of logiclevels and translates the input signals to a first pair of logic levelsfor use by circuitry in the nonvolatile memory, wherein the low-powerinterface provides output signals at the same pair of logic levels asthe input signal, wherein the low-power interface comprises:an inputbuffer coupled to the nonvolatile memory and coupled to a first powersupply, the input buffer accepting input signals having a second pair oflogic levels; an output buffer coupled to the nonvolatile memory andcoupled to a second power supply, the output buffer providing outputsignals having a second pair of logic levels.
 2. The nonvolatile memoryof claim 1, wherein the first and second pairs of logic levels arecompatible with complementary metal-oxide semiconductor (CMOS)technology.
 3. The nonvolatile memory of claim 1, wherein the outputbuffer is coupled to the first power supply output.
 4. The nonvolatilememory of claim 1, wherein the input buffer translates the second pairof logic levels of the input signal to the first pair of logic levels.5. The nonvolatile memory of claim 1, wherein the first pair of logiclevels includes a logic high signal substantially equal to 3.0 volts anda logic low signal substantially equal to 0 volts.
 6. The nonvolatilememory of claim 5, wherein the second pair of logic levels includes alogic high signal substantially equal to 1.8 volts and a logic lowsignal substantially equal to 0 volts.
 7. The nonvolatile memory ofclaim 6, wherein the first power supply output provides a voltagesubstantially in the range from 2.7 volts to 2.85 volts.
 8. Thenonvolatile memory of claim 6, wherein the second power supply outputprovides a voltage substantially in the range from 1.8 volts to 2.2volts.
 9. The nonvolatile memory of claim 5, wherein the second pair oflogic levels includes a logic high signal substantially equal to 3.0volts and a logic low signal substantially equal to 0 volts.
 10. Thenonvolatile memory of claim 9, wherein the first and second power supplyoutputs each provide a voltage substantially equal to 3.0 volts.
 11. Thenonvolatile memory of claim 1, wherein the input buffer and the outputbuffer include a high transconductance n-channel transistor, thetransistor doped to provide a threshold voltage that is lower than astandard n-channel transistor.
 12. The nonvolatile memory of claim 1,wherein the input buffer comprises a p-channel transistor feedbackpull-up device, the feedback pull-up device preventing the input bufferfrom drawing leakage current.
 13. The nonvolatile memory of claim 1,wherein the first and second power supply outputs are outputs from asingle power supply.
 14. A method of handling signals associated with anonvolatile memory, the signals having a plurality of logic levels, themethod comprising the steps of:translating a first pair of logic levelsof a signal input to the nonvolatile memory to a second pair of logiclevels when the first pair of logic levels are different from the secondpair of logic levels; translating the second pair of logic levels of asignal output from the nonvolatile memory to the first pair of logiclevels when the second pair of logic levels are different from the firstpair of logic levels.
 15. The method of claim 14, wherein the first andsecond pairs of logic levels are compatible with complementarymetal-oxide semiconductor (CMOS) technology.
 16. The method of claim 14,wherein an input buffer receives the input signal and translates thefirst pair of logic levels of the input signal to the second pair oflogic levels.
 17. The method of claim 16, wherein the system processorand the input buffer and the nonvolatile memory are coupled to a firstpower supply output.
 18. The method of claim 17, wherein the first powersupply output provides a voltage substantially equal to 3.0 volts andthe first pair of logic levels includes a logic high signalsubstantially equal to 3.0 volts and a logic low signal substantiallyequal to 0 volts.
 19. The method of claim 17, wherein the first powersupply output provides a voltage substantially in the range from 2.7volts to 2.85 volts and the first pair of logic levels includes a logichigh signal substantially equal to 1.8 volts and a logic low signalsubstantially equal to 0 volts.
 20. The method of claim 14, wherein anoutput buffer provides the output signal, the output buffer coupled to asecond power supply output.
 21. The method of claim 20, wherein thesecond power supply output provides a voltage substantially equal to 3.0volts and the first pair of logic levels includes a logic high signalsubstantially equal to 3.0 volts and a logic low signal substantiallyequal to 0 volts.
 22. The method of claim 20, wherein the second powersupply output provides a voltage substantially in the range from 1.8volts to 2.2 volts and the first pair of logic levels includes a logichigh signal substantially equal to 1.8 volts and a logic low signalsubstantially equal to 0 volts.
 23. The method of claim 14, wherein thesecond pair of logic levels includes a logic high signal substantiallyequal to 3.0 volts and a logic low signal substantially equal to 0volts.
 24. The method of claim 23, wherein the first pair of logiclevels includes a log ic high signal substantially equal to 1.8 voltsand a logic low signal substantially equal to 0 volts.
 25. The method ofclaim 23, wherein the first pair of logic levels includes a logic highsignal substantially equal to 3.0 volts and a logic low signalsubstantially equal to 0 volts.
 26. The method of claim 20, wherein thefirst and second power supply output are outputs from a single powersupply.